In recent years, a digital broadcasting by a satellite or a terrestrial wave has generally been spread. On the other hand, a transmission of digital content data such as video image, audio sound, or the like via a broadband home communication network has been used.
According to those services, generally, the content such as video image, audio sound, or the like is compressed by a system such as MPEG or the like and transmitted in a format of a stream or a file. In a receiver, it is decompressed by a decoder and played back. If the receiver has a storing function, the received content is stored and may be played back as necessary later.
In the case of a service for providing a content such as video image, audio sound, or the like for a charge, there is a case where the content has been encrypted. When the content is read out from the storing function and played back, the encryption is decrypted by using key information obtained separately, thereby playing back the content (for example, refer to patent document 1).
Patent document 1: JP-A-2003-174439